Why don't two 50% lights sum up as 100% in linear mode, but do in srgb, simply making two overlaping spots to test shows each light at 50% in both cases, but the sum is different... I have read hellolux article but there is no obvious answer there, or atleast not for me, can anyone explain?

I've dug up this graph showing the sRGB values vs sRGB intensities, from here. The red line is the one.

50% on the x is close to 35% on the y. So it looks like there's a LWF conversion going on in calculating the combined brightnesses of two lights.

Not going to pretend to understand if this is 'correct' in physical terms - or if that's even a valid question. What is '50% brightness' for a light? Is a 50W bulb half as bright as a 100W bulb? If you overlap two 50W spot lights, do you get brightness equivalent to a 100W bulb in the space where they overlap? But the maths seems to point to the answer being here somewhere:

Where the lights intersect would be some distance from the origin, and so I would think falloff would be a factor, so the sum of the brightness where they intersect would be less than the sum of the brightness at the source. Just a guess. . .

When using linear color space there is no need for gamma correction. The problem with sRGB is that 50% are not 50% To get both lights added up to 100% you need to set the lights to 72.974% (50% ^ 1/2.2).

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